Possible but very unlikely. My uncle went to Heathrow airport a few months ago without a confirmed reservation (full fare Y ticket) hoping to standby for EK030 (Dep 1700.) He waited from 1400 to 1700 but could not get on. However, he was able to get a seat in Y for EK004 (Dep 2000.) He made a fuss and said the least they can do is upgrade him cause he had to wait for so long (even though it was completely his fault not EK's ) and they obliged and gave him a J seat. Flight was full in F&J, some seats in Y. He is skywards gold

Cheers